Superalloys are heat resistant materials having superior strength and oxidation resistance at high temperatures. Many of these alloys contain iron, nickel or cobalt alone or in combination as the principal element, together with chromium to impart surface stability and usually containing one or more minor constituents such as molybdenum, tungsten, columbium, titanium and aluminum for the purpose of effecting strengthening. The physical properties of the superalloys make them particularly useful in the manufacture of gas turbine components.
The strength of superalloys is determined in part by their grain size. At low temperatures fine grained equiaxed structures are preferred. At high temperatures large-grained size structures are usually found to be stronger than fine grained. This is believed related to the fact that failure generally originates at grain boundaries oriented perpendicular to the direction of the induced stress. By casting a superalloy to produce an elongated columnar structure with unidirectional crystals aligned substantially parallel to the long axis of the casting, grain boundaries normal to the primary stress axis can be almost completely eliminated. Further, by making a single crystal casting of a superalloy, such failure under stress is entirely eliminated.
Directional solidification to produce columnar casting and the apparatus used for this purpose are described in The Superalloys, edited by C. T. Sims et al., John Wiley & Sons, (1972), pages 479-508. Columnar grains are formed in a casting where the flow of heat is unidirectional from the liquid through the solid.
The temperature gradient at the solid-liquid interface of a casting in any directional solidification apparatus is a major factor which regulates the maximum rate unidirectional solidification can occur while maintaining good phase alignment throughout the length of the ingot. An increase in growth velocity requires an increase in temperature gradient in order to maintain the same ratio of temperature gradient to growth velocity.
Typically a ceramic investment casting mold is attached to a water-cooled copper chill plate and placed in an induction-heated graphite susceptor or resistance heated furnace. The mold is heated above the melting point of the alloy being cast and a superheated melt is poured into the mold. Heat enters the upper portion of the mold by radiation from the susceptor or other heat source and is removed through the solidified metal by the chill at the bottom. Thus, solidification occurs in an upward direction through the casting and the rate of solidification is a function of the amount of heat entering at the top of the casting and the amount of heat extracted from the casting through the solid. Alternatively, the charge can be melted in situ, within the mold, by the furnace. After equilibrium is established, the mold assembly is lowered out of the heat zone and nucleation of solid occurs in the bottom of the mold. Directional freezing continues upward as the mold unit is lowered. Faster rates at this inherent temperature gradient introduces structure breakdown to cellular and/or dendritic morphologies which deleteriously affects the properties. Bottomless crucibles which allow contact between the ingot and a copper chill have increased the allowable solidification rate but the heat path may still be interrupted by oxide formation at the contact site or poor contact between the ingot and the chill due to surface roughness, lack of alignment or separation due to shrinkage of the ingot during cooling.
The conditions at the chill face are critical for proper unidirectional heat flow. The chill should be water cooled and have a high thermal conductivity. The surface of the chill must be cleaned before each casting run so that resistance to heat flow by oxide layers is minimized. Difficulties in obtaining uniform heat transfer at the chill face require that the mold be securely clamped to the chill plate. A major problem with this method is that solidification rate and temperature gradient decrease with distance from the chill.
In accordance with the invention described in U.S. Pat. No. 3,939,895, a method is provided of producing a directionally solidified cast alloy article in a shell mold. The method includes providing a mold having a cavity divided into an upper portion and a lower portion, the mold being disposed in a heating zone, placing one end of a longitudinal heat extractor element of said alloy into the lower portion of the cavity, said other end of said heat extractor extending therefrom and being exposed to a continuous flow of fluid coolant, heating said mold and said one end of said heat extractor placed therein at a temperature above the melting range of said alloy to melt a portion of said one end of the heat extractor, filling the mold with said alloy in a molten state and controllably lowering said mold out of the heating zone to allow the mold and contents thereof to cool and to establish directional solidification of the alloy in said cavity.